From pretty early on, Oz was established as having two major rivers, the Munchkin and Winkie, both of which flow into a lake outside the Emerald City, later named Lake Quad. Whether this means the city is on low ground probably isn’t something we should assume (ozume?) in Oz. I don’t recall when the Gillikin River is first named, but according to the map by James E. Haff and Dick Martin, it’s where the Scooters live and where Kite Island is located. The Quadling River isn’t mentioned until The Ozmapolitan of Oz, and even there it isn’t visited or specifically located. The best bet might be the river on which the Great Waterfall is located. The waterfall itself ends in a pool, and the Scarecrow explains in his own book, “Only a pool of whirling water showed what had become of the river; but I suppose it is under ground, somewhere, and will come to the surface again in another part of the country.” The map refers to the waterfall itself as the Magic Waterfall, but there’s no indication in the text that it’s magical, just that it’s the largest waterfall in Oz.
There’s also the river that Robin Brown and Merry Go Round have to cross in Merry Go Round, identified on the map in that book as the Red River, and also near a waterfall.
While it would make a certain amount of sense for the Quadling River to empty into Lake Quad, I’m not sure exactly what path it would take. I think of it as mostly running in the eastern part of the Quadling Country, and I placed my own original locations of Yartralia, Hyrystalia, and the Red Desert on it.
More minor but still noteworthy rivers include the Trick River that repeatedly changes its current, the River Road in the Gillikin Country that suddenly turns from a road into a river, and the Cream River. The Black River in the Black Forest can dye anyone black, including their eyes, allowing them to see in the dark. In Lucky Bucky, Davy Jones enters Oz proper in a lake that flows into the Stiff River, which is totally straight except when it turns at right angles.
It runs through a very neat country, and has strict rules including one against drowning except on Tuesdays. It’s also the home of cattails connected to actual cats, which the Flummux suggests might be victims of an evil enchantment.
The Stiff River ends abruptly, but nearby is the Game River, then a river that flows through a dangerous country inhabited by rogue magicians. At this point in history, the Wizard of Oz has rolled up the rivers in the more inhabited parts of Oz due to their bringing witches and monsters into civilized lands, but he presumably restores them eventually, as per the plot of Ozmapolitan. I deal a bit with this in my own story “Revolt of the Scalawagons.” The Scarecrow keeps one rolled-up river in his basement, and Davy uses it to reach Lake Quad, dragging the river along behind him. It also turns out to have water that won’t let anyone sink. The Singing Brook in the Lolly-Pop Village is home to water fairies and kelpies, and Ozma and her friends have a party there during Scalawagons.
Two frogs run a bathing pavilion with bathing suits and towels, as well as refreshments.
Forbidden Fountain has Camouflage Creek, which makes anyone who bathes in it turn into whatever sorts of animals are in the area. Some of the characters in Gina Wickwar’s Hidden Prince have to cross the quite literal Babbling Brook, which flows into the Caterwauling Cataracts.
Anyone crossing has to wait for a drawbridge consisting of a giant hand holding a pencil and drawing a bridge. It will then proceed to erase it, not always waiting for everyone to get across.
A minor character in the story is a bridge that ran away from its original river because it felt unappreciated, and later decides to span another river that’s too ticklish for anyone to wade across it.
The idea of a bridge willfully leaving its river had previously appeared in Wonder City, when Ozma’s procession had to make their own bridge over Cream River because the regular one is out.
The most detailed description of Lake Quad is in a fragment known simply as “An Oz Book,” credited to L. Frank Baum but not really in his style. Ozma, by means of some faucets, can cause the lake to freeze solid or to shift colors. At the bottom is a pipe that drains the water into the Nonestic Ocean at the Kingdom of Rinkitink. When this fragment became the basis for a round-robin story that was published in the 2012 Oziana, at least one of the writers suggested this lake might have been different from Lake Quad. Regardless, there’s presumably some outlet from Lake Quad to the ocean, and David Hulan’s Glass Cat makes it accessible by mermaids from the sea.
It’s a freshwater lake, but there are at least two salt lakes in Oz: Lake Orizon, where the Ozure Isles and Sapphire City are located; and the inland sea that Mombi temporarily turns into gelatin in Lost King. I would imagine the Soup Sea also contains a good amount of salt, as well as pepper. Other lakes in Oz include the yellow one in the Winkie Country where Yankee’s space capsule landed, Lake Lily with its beautiful water nymph, that of the Skeezers, the soapy lake at the edge of Suds, the small lake of ink near Sign-Here known as the Link, and the Lemonade Lake that’s referenced in at least one issue of the Ozmapolitan. Wickwar’s Toto of Oz largely takes place in the Lakes District of the Gillikin Country.
And while they’re not big enough to be considered lakes, I suppose the Truth Pond and the pool where the magic skosh that enlarged the Frogman grows are worthy of note.
There are fewer notable bodies of water in the lands surrounding Oz, but some are worth mentioning. The Land of Ev has the dry river that runs through the Rubber Country, as well as river with the irritating Tickley Bender as its head.
Apparently the phrase “tickly bender” refers to a thin patch of ice, as well as games that kids played on one of these patches. I wonder if Tickley was responsible for the plight of the ticklish river from Hidden Prince. The main river in Merryland flows around the whole country in more or less a circle, flowing through all seven valleys. It also connects to a river near Syracuse, New York; or at least it did at one point. Mo has rivers of root beer, maple syrup, milk, and needles. The Milk River has cheese islands and pools of cream, while Rootbeer River is the location of Fruitcake Island.
Lightning Lake can be found inside Thunder Mountain, and The Life and Adventures of Santa Claus mentions the Pool of Nares in the Forest of Burzee.